The radio frequency identification (RFID) industry employs a number of frequencies and modulation techniques to communicate information between cards and readers. The reader comprises a crystal and additional electronics that serve to generate a carrier signal of usually one hundred twenty-five (125) kHz. The access control portion of the industry has historically employed the carrier signal frequency to power passive credit-card-size cards that communicate codes back to the reader. The magnetic field generated by the carrier signal frequency serves to power, at a range of a few inches or several centimeters, the credit-card-size cards that comprise a coil resonated by a capacitor and electronics such as an integrated circuit (IC) powered and clocked by the magnetic field. The electronics of the card responds to the magnetic field and loads coded information from the card onto the magnetic field in a time dependent manner that is determined by the card electronics. The card communicates a unique serial code of usually thirty-two (32) to two hundred fifty-six (256) bits back to the reader to establish the identity of the holder or owner of the card.
The card employs modulation techniques that depend on the particular manufacturer of the equipment. The various manufacturers of access control equipment have chosen differing modulation techniques to transfer information in a bit stream from the cards to the reader. Common modulation techniques employed with access control cards comprise: direct modulation of the carrier signal in amplitude-shift keying (ASK); frequency-shift keying (FSK) modulation of a sub-carrier signal generated by the electronics of the card; or phase-shift keying (PSK) modulation of a sub-carrier signal generated by the electronics of the card. The electronics of the card typically employs digital division of the carrier signal frequency to generate the bit rates and sub-carrier signal frequencies. The electronics of the card usually imparts additional encoding and bits to the serial code such as Manchester and/or bi-phase encoding, and/or parity bits and/or check sums, to provide a serial card data stream as an output.